This application claims priority under 35 USC 119(a) to Patent Application No. 2004-095915 filed in Japan on 29 Mar. 2004, the content of which is hereby incorporated herein by reference in its entirety.
The present invention relates to a charging unit such as might be employed in a copier, laser printer, facsimile machine, or other such electrophotographic image forming apparatus, and to an image forming apparatus of the type in which corona-type charging unit(s) or other such charging unit(s) is/are used to charge photosensitive body surface(s).
Copiers and other such electrophotographic image forming apparatuses include monochromatic image forming apparatuses which form black-and-white images, and color image forming apparatuses which form a color images. Color image forming apparatuses include multirotational-type image forming apparatuses in which toner images corresponding to respective color components (black, cyan, magenta, and yellow) are sequentially formed on a single photosensitive body by way of toner image forming means for the respective colors, and tandem-type image forming apparatuses in which a plurality of toner image forming means respectively forming toner images corresponding to the respective color components more or less simultaneously on separate photosensitive bodies are arranged in series in the transport direction of a transfer intermediary (see, e.g., Japanese Patent Application Publication Kokai No. 2003-191526).
Moreover, tandem-type image forming apparatuses include apparatuses of the type permitting selection of either a full-color printing mode or a monochromatic printing mode (machines capable of both color and black-and-white). Furthermore, of the machines that are capable of both color and black-and-white printing, there are apparatuses permitting switching between/among processing speeds, it being possible to choose processing speed(s) when carrying out monochromatic printing that is/are faster than the processing speed(s) available when carrying out color printing.
Moreover, as charging units in electrophotographic image forming apparatuses, corotron charging units employing wires and cases, scorotron charging units employing wires (discharge electrodes) and cases together with grid electrodes to stabilize surface potentials at photosensitive bodies, and other such corona-type charging units are frequently used (see, e.g., Japanese Patent Application Publication Kokai No. 2002-229302). In particular, scorotron charging units possess the advantage that arrangement of grid(s) between discharge electrode(s) and photosensitive body surface(s) permits stable control of photosensitive body surface potential(s). However, with corona-type charging units, there is the problem of the ozone emissions that accompany corona discharge.
Moreover, among copiers and other such image forming apparatuses, there are models having different processing speeds, e.g., there are slow machines, fast machines, and so forth; and because charging units have conventionally been designed such that each model has had its own dedicated charging unit(s), this has not only resulted in high cost but has also been a problem in terms of inventory/turnaround.
Furthermore, where it is possible to switch between/among processing speeds in a single color image forming apparatus, to permit faster processing speed(s) when carrying out monochromatic printing as described above, the charging unit(s) used for black have been larger (e.g., corona-type charging unit(s) having larger case width(s)) than the charging unit(s) for the other colors (cyan, magenta, and yellow). This being the case, it has been difficult to make use of common charging unit(s) within the same image forming apparatus. In addition, because frequency of usage of black charging units is high, charging characteristics of black charging units tend to deteriorate rapidly.